What is the difference between an electromagnet and a permanent bar magnet?

Electromagnets 01

This KS3 Science quiz takes a look at electromagnets. When electricity passes through a wire, it creates a weak magnetic field. If the wire is coiled into a spiral shape, the magnetic field from the wire is stronger. If the coil is wrapped round an iron core e.g. a nail, the magnetic field is made even stronger. But it doesn't stop there ... if you increase the current flowing through the coiled wire, the magnetic effect is also increased. Finally, if you add more coils to the spiral, the magnetism can be increased yet again! A coil of wire round an iron core that is connected to a source of electricity is called an electromagnet.

Electromagnets have two big advantages over permanent magnets - they can be switched on and off and they can be made to be very powerful. Three important uses are in electric motors, electric bells and relays.

A relay is a switch that is operated by electricity. It consists of a springy switch contact which is set up to be attracted by an electromagnet. When the electromagnet is switched on or off, the contact arm is attracted and the switch is operated. A relay is a good safety device as a low voltage circuit can be used to operate the electromagnet which then will operate the switch connected to a high voltage circuit. There is therefore no danger of the person operating the electromagnet coming into contact with the high voltage electricity.

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They can also be made a lot stronger and for less cost than a permanent magnet of the same strength

2.

The magnetic field around a coil of wire carrying current:

Is a circular shape

Is a diamond shape

Is a square shape

Is the same shape as around a bar magnet

A coil of wire is known as a solenoid

3.

Which of the following is a difference between an electromagnet and a permanent bar magnet?

Electromagnets are stronger

Electromagnets can be turned off

Permanent magnets are stronger

Permanent magnets can be turned off

It is that feature that makes them so useful

4.

Which of the following is NOT true about the magnetic field around a coil of wire carrying a current?

Changing the direction of the current will reverse the poles

Increasing the number of coils will increase the strength

Making the coil wider will increase the strength

Putting an iron core into the coil will increase the strength

Making the coil wider would make it shorter, unless you used more wire

5.

Which of the following uses an electromagnetic effect?

Iron

Light bulb

Toaster

Washing machine

Electromagnetic effect is used in any appliance with a motor

6.

A relay is used in:

A car ignition

An electric bell

An electric cooker

An iron

A starter motor needs a large current

7.

If the current direction through the wire is reversed, the direction of the magnetic field:

Is also reversed

Stays in the same direction

The current stops flowing

There is no magnetic field

Whenever there is a current flowing, an electromagnet will have a magnetic field around it

8.

A relay uses a small current to:

Attract a magnet

Repel a magnet

Switch off a current

Switch on a larger current

When you press a lift button, it operates a relay that switches on the motor. The motor needs a high current and if that sort of current passed through a lift call button, it would be dangerous if something went wrong

9.

If the current in the wire increases, what happens to the strength of the magnetic field?

It decreases

It increases

It stops the magnetic field

No change

There is a direct positive correlation between current and magnetic field strength

10.

The magnetic field around a straight wire carrying an electric current has: